1. Field of the Invention
The present invention relates to ultrasonic sheet material testing devices, and more particularly to ultrasonic rolling transducers and to testing devices of the type which measure the time taken by an ultrasonic signal to travel through a length of the sheet material.
2. Description of the Prior Art
A number of prior art rolling contact ultrasonic transducers are known. However, none of these rolling transducers are capable of introducing ultrasonic signals into sheet material for travel generally in the plane of the material from the location of entry to another location. Therefore, they cannot be used in a device which measures the travel time of an ultrasonic signal along a length of sheet material for use in quality testing the material. An example of a typical known rolling transducer is shown in U.S. Pat. No. 3,628,378 of Pagano. In Pagano, ultrasonic signals are transmitted from a transducer in directions normal to, or offset somewhat from normal to, the surface of a test material so that they are reflected back from the material. Echoes of the transmitted signals returning to the transducer are then displayed on a cathode ray tube oscilloscope.
In many industrial plants, sheet material is graded by an employee who watches the material as it passes by on a conveyor. This method of grading is highly inaccurate since the visually-observable characteristics relate poorly to mechanical properties of the material. Factors affecting accuracy include the fatigue of the employee, limitations on the ability of the employee to discover concealed flaws such as hairline cracks and internal defects, the experience of the employee, and the speed at which the sheet material is conveyed.
In an effort to make sheet material testing more accurate, one prior art device introduces stress waves into a sheet of particleboard using a mechanical impactor clipped to one end. A sensor clipped to the other end of the sheet receives these waves and the time taken for the waves to travel through the sheet is indicated on a meter. Although in some instances this type of device will produce a quality indication which is more accurate than can be obtained by an employee using the above approach, it is extremely time consuming. That is, the impactor and sensor are first manually clipped to a stationary panel and a time measurement is obtained. To take another reading on the same panel, or on another panel, these clipped elements must be moved. Because of the time required to take each measurement, it is not feasible to take more than a relatively few measurements for each panel. As a result, accuracy suffers as flaws that do not happen to be in the path of a stress wave won't be detected. More importantly, it would be impractical to try and clip the sensor and the impactor to sheet material moving on a conveyor. Furthermore, this type of device is monitored by an operator who compares a measured stress wave travel time measurement with a known standard travel time. For this reason, operator fatigue and the other factors mentioned above also affect overall accuracy and suitability as a testing system.
Accordingly, there is a need for a rolling contact transducer capable of accurately focusing ultrasonic waves into sheet material for travelling in the sheet and for such a transducer which detects ultrasonic signals travelling in the sheet. There is also a need for an ultrasonic testing device which measures the travel time of ultrasonic signals through a length of sheet material and uses these measurements in grading the sheet.